The Universal Mobile Telecommunication System (UMTS), also referred to as the third generation (3G) system or the wideband code division multiplexing access (WCDMA) system, is designed to succeed GSM. UMTS Terrestrial Radio Access Network (UTRAN) is the radio access network of a UMTS system.
High-Speed Downlink Packet Access (HSPDA) is an evolution of UTRAN bringing further enhancements to the provisioning of packet-data services both in terms of system and end-user performance. The downlink packet-data enhancements of HSDPA are complemented with Enhanced Uplink (EUL), also known as High-Speed Uplink Packet Access (HSUPA). EUL provides improvements in the uplink capabilities and performance in terms of higher data rates, reduced latency, and improved system capacity, and is therefore a natural complement to HSDPA. HSDPA and EUL are often jointly referred to as High-Speed Packet Access (HSPA).
FIG. 1 illustrates a wireless communications system, such as a HSPA system, including a radio access network such as the UTRAN. The UTRAN architecture comprises at least one radio base station (NodeB) 130, connected to one or more Radio Network Controllers (RNC) 100. The UTRAN is connected over an interface to a Core Network (CN) 120. The UTRAN and the CN 120 provide communication and control for a plurality of user equipments (UE) 150. The UEs 150 are wirelessly connected to at least one NodeB 130, and they communicate with each other over downlink and uplink channels.
In a UTRAN, the dedicated transport channel is called Dedicate Channel (DCH). The DCH carries all the information to/from a specific UE from/to higher layers including the data for the actual service and higher layer control information. In a UTRAN with HSPA, the HSPA enhancements are implemented through new dedicated transport channels: the High-Speed Downlink Shared Channel (HS-DSCH) for HSDPA and the Enhanced Dedicated Channel (E-DCH) for EUL.
Packet-data is often transmitted in bursts, which gives occasional periods of transmission activity and no transmission activity in between these periods. From a user performance perspective, it is advantageous to keep the HS-DSCH and E-DCH configured to rapidly be able to transmit any user data. At the same time, maintaining the connection in uplink and downlink comes at a cost. From a network perspective, there is a cost in uplink interference from the control data transmission even in absence of data transmission. From a UE perspective, power consumption is the main concern; even when no data is received the UE needs to transmit and monitor control data. To reduce UE power consumption, UMTS/WCDMA has several connected mode states which define what kind of physical channels a UE is using: Cell_DCH 220, Cell_FACH 210, and Cell_PCH/URA_PCH 200, schematically illustrated in FIG. 2, with arrows defining the possible state changes.
The lowest power consumption is achieved when the UE is in one of the two paging states Cell_PCH/URA_PCH 200. For exchange of data, the UE needs to be moved to the Cell_FACH 210 or Cell_DCH 220 state. The high transmission activity state is known as CELL_DCH 220. In this state, a dedicated physical channel is allocated to the UE and the UE can use e.g. HS-DSCH and E-DCH for exchanging data with the network. This state allows for rapid transmission of large amounts of user data, but also has the highest UE power consumption. In order to avoid a waste of UE battery, the UE is switched to CELL_FACH 210 state if there hasn't been any transmission activity for a certain period of time. In Cell_FACH 210 state, the UE can transmit small amounts of packet data as part of the random access procedure on the Random Access Channel (RACH). The UE also monitors common downlink channels (Forward Access Channel (FACH)) for small amounts of user data and Radio Resource Control (RRC) signalling from the network.
The RACH is an uplink transport channel intended to be used to carry control information from the UE, such as requests to set up a connection. RACH is mapped on the Physical Random Access Channel (PRACH). As stated above, the RACH procedure can also be used to send small amounts of packet data from the UE to the network in the Cell_FACH state.
The following paragraphs outlines the Layer 2 (L2) Medium Access Control (MAC) description for controlling the transmissions in the RACH procedure, as described in the 3GPP (3rd Generation Partnership Project) technical specification TS 25.321. The RACH transmissions are controlled by the UE MAC sublayer, hereinafter referred to as UE (MAC). The UE (MAC) receives the following RACH transmission control parameters from the Radio Resource Control (RRC):                a set of Access Service Class (ASC) parameters, which includes for each ASC, i=0, . . . ,NumASC an identification of a PRACH partition and a persistence value Pi (transmission probability);        maximum number of preamble ramping cycles Mmax;        range of back-off interval for timer TBO1, given in terms of numbers of transmission time (10 ms) intervals NBO1 max and NBO1 min.        
When there is data to be transmitted, the UE (MAC) selects the ASC from the available set of ASCs, which consists of an identifier i of a certain PRACH partition and an associated persistence value Pi. Based on the persistence value Pi (used in a persistency test), the UE decides whether to enter the Layer 1 (L1) PRACH transmission procedure in the present transmission time interval (TTI) or not. If transmission is not allowed, a new persistency test is performed in the next TTI, and the persistency test is repeated until transmission is allowed. When transmission is allowed, the PRACH transmission procedure—starting with a preamble power ramping cycle—is entered. The UE (MAC) then waits for access information from L1.
When the preamble has been acknowledged on the Acquisition Indicator Channel (AICH), L1 access information with parameter value “ready for data transmission” is indicated to the UE (MAC). The data transmission is then requested, and the PRACH transmission procedure is completed with transmission of the PRACH message part according to L1 specifications. When no acknowledgement on AICH is received while the maximum number of preamble retransmissions is reached, a new persistency test is performed in the next TTI. The timer T2 ensures that two successive persistency tests are separated by at least one 10 ms time interval.
If a negative acknowledgement is received on AICH, a back-off timer TBO1 is started. After expiry of the timer, a persistency test is performed again. Back-off timer TBO1 is set to an integer number NBO1 of 10 ms time intervals, randomly drawn within an interval 0≤NBO1 min≤NBO1≤NBO1 max (with uniform distribution). NBO1 min and NBO1 max may be set equal when a fixed delay is desired, and even to zero when no delay other than the one due to the persistency test is desired.
Before a persistency test is performed it shall be checked whether any new RACH transmission control parameters have been received from RRC, and the latest set of RACH transmission control parameters shall be applied. If the maximum number of preamble ramping cycles Mmax is exceeded, failure of RACH transmission shall be reported to higher layer.
The RACH procedure has to cope with the near-far problem, as there is no exact knowledge of the required transmission power when entering the transmission procedure. As indicated above, this is solved with a preamble transmission procedure with power ramping. In the following, a summary of the different UE steps of a RACH procedure will be described with reference to FIG. 3a. 
The UE decodes the broadcast channel 301 to find out the available RACH sub-channels and their scrambling codes and signatures, as well as the transmission control parameters (see above). Based on the persistence value Pi, the UE decides whether to enter the transmission procedure in the present TTI or not. This so called persistency test 302 is explained with more details below. If the persistency test allows transmission 303/YES, the UE selects randomly one of the RACH sub-channels. If the persistency test does not allow transmission 303/NO, the UE needs to wait for the next TTI before a new persistency test 302 is performed. This is repeated until transmission is allowed.
The downlink power level is measured and the initial RACH power level is set 304 based on this measurement (according to the open loop power control). A first preamble is transmitted 305. The UE decodes the Acquisition Indicator Channel (AICH) 306 to see whether the NodeB has detected the preamble. In case no AICH is detected 306/NO, the UE increases the preamble transmission power 304 by a step given by the NodeB. The preamble is retransmitted 305 in the next available access slot. If the maximum number of preambles has been reached, a new persistency test 302 is performed.
When an acknowledgement (ACK) from the NodeB is detected on AICH 306/ACK, the UE transmits the message part of the RACH transmission 307. In the case of a blocking situation (e.g. two UEs requesting a connection at the same time) the NodeB will transmit a NACK on the AICH 306/NACK to one of the UEs. This will force the UE to exit the RACH procedure and re-enter it after a certain delay controlled by the timer TBO1 308. After expiry of the timer, a new persistency test 302 is performed to check if the UE is allowed to re-enter the procedure.
During the persistency test 302, referred to above, the UE generates a random value between 0 and 1 and checks whether this value is within the interval given by the persistency value Pi. A UE generating a random value below a threshold defined by the persistency value Pi, will be allowed to start the RACH procedure. By configuring the persistency value parameter, the probability of a UE entering the preamble transmission procedure can be controlled. As an example, if the persistency value is set to 0.9, there is a 90% probability that the UE will initiate the RACH procedure, which means that the delay is typically rather short, while with a persistency value of 0.1, there is only a 10% chance of the UE initiating the procedure, thus typically giving a longer delay.
The network steps in the RACH procedure are described below with reference to FIG. 3b. The RNC configures the transmission control parameters and transmits them via layer3 signalling. The NodeB broadcasts the available RACH sub-channels and their scrambling codes and signatures, as well as the transmission control parameters 311. When the UE has reached the needed preamble transmission power level, the NodeB will receive the preamble 312. NodeB will then check for available resources 313, and will transmit an ACK and the resource allocation 314 when resources are available 313/YES. After having received the message part of the RACH transmission 315, the resources will be released by the NodeB 316. If the resource availability check is negative 313/NO, a NACK will be transmitted on the AICH instead.
In the 3GPP, the transmission procedure in the Enhanced Uplink in CELL_FACH state has been discussed, and it has been agreed to use a preamble transmission procedure with power ramping with the same transmission control parameters as in the ordinary RACH procedure (as described above), and to use a specific AICH or EUL AICH (E-AICH) sequences indicating EUL resources to the UE. This procedure will hereinafter be referred to as Enhanced Uplink in CELL_FACH state procedure.
A disadvantage of this solution, is that the delay for the UEs to access the EUL resources in the Enhanced Uplink in CELL_FACH state procedure, is the same as the delay for the UEs to access ordinary UL resources in the RACH procedure. Since both procedures serve quite different purposes, an equal delay will give sub-optimal performance of the Enhanced Uplink in CELL_FACH state procedure. Solutions for a reduced delay for the Enhanced Uplink in CELL_FACH state procedure has been discussed in 3GPP, and it has been proposed to re-enter the preamble transmission with the power level of the latest preamble transmission before NACK, alternatively with the power level minus a small power back-off of the latest preamble transmission before NACK.